Photonic crystals are materials having a periodic modulation in their refractive index (Yablonovitch, Phys. Rev. Lett., 58:2059, 1987). This periodic modulation creates a photonic band gap that either attenuates or is permissive to the propagation of electromagnetic waves of particular wavelengths. Band gap positioning is defined by the distance between the periodic modulations in the crystal. The reflected stop band wavelengths can appear in the reflectance spectrum as a distinct reflectance peak known as a Bragg peak. The crystal may have a one-, two-, or three-dimensional (3D) periodic structure.
Similar devices known as phononic crystals have a phononic band gap, which is the acoustic analog of a photonic band gap. In a phononic crystal, it is a desired range of acoustic frequencies that cannot exist in the structured material. Depending on the material used, a particular crystalline structure can be adjusted to create a bandgap for waves of either photonic or phononic nature. Such crystals are known as phoxonic crystals.
Bandgaps in phononic or photonic crystals are created by the presence of periodic scatter material in a homogeneous host matrix that propagates an energy wave. This scattering material sets up wave interference. If the interference is destructive, the energy of the wave is negated, and the wave cannot propagate through the crystal. It is this destructive interference that creates the bandgap. Minor changes in the lattice spacing of a photonic or phononic crystal produce easily detectable shifts of the reflected stop band. Examples of tuning the bandgap during crystal formation can be found in U.S. Patent Application Publication No. 2004/0131799, and U.S. Pat. No. 7,826,131.
External stimuli such as physical deformation, swelling with a solvent material, or application of a potential can be used to alter the periodic spacing of the crystal structure. A tunable crystal by deformation of an already formed material to create a loose packed structure of spheres embedded in a hydrogel or elastomer matrix has been done as is found in U.S. Patent Application Publication Nos. 2011/0222142 and 2011/0014380, U.S. Pat. Nos. 7,826,131, 7,538,933, 6,544,800, 5,266,238, 5,368,781, and in publications of H. Fudouzi and Y. Xia, “Photonic Papers and Inks: Color Writing with Colorless Materials,” Advanced Materials, 15(11), 892-896, 2003, H. Fudouzi and Y. Xia, “Colloidal crystals with tunable colors and their use as photonic papers,” Langmuir, 19, 9653-9660, 2003, H. Fudouzi and T. Sawada, “Photonic rubber sheets with tunable color by elastic deformation,” Langmuir, 22(3), 1365-8 (2006), Holtz et al., Nature 389:829-832, Foulger et al., Advanced Materials 13:1898-1901, Asher et al., Journal of the Material Chemical Society 116:4997-4998, and by Jethmalani et al. Chemical Materials 8:2138-2146.
These post-formation tuning methods, however, are reversible so as to be considered temporary. For example, it is known to adjust the bandgap of a photonic crystal by swelling a colloidal particle array embedded in a matrix using a solvent. Fudouzi and Xia formed photonic crystal papers that would swell in the presence of an ink or solvent such as 1-propanol. (Langmuir, 2003; 19:9653-9660.) Upon evaporation of the solvent, the paper returned to its original position.
The reversibility of prior crystal materials negates their usefulness for long term applications where loose packed crystal structures are desired. As such, new methods are needed for the cost effective and rapid formation of loose ordered crystal structures of periodic material.